Nearly 4000 lightning strikes over NZ; concentrated barrage for Auckland

Thursday, 6 June 2019

Watch as lightning strikes the tip of the Sky Tower during an Auckland storm.

Nearly 4000 lightning strikes were recorded in the 24 hours to 9am Thursday, most in the top half of the North Island, including an intense barrage over Auckland during Wednesday's storm.

Out of a total of 3979 strikes counted by MetService, 838 were over land, with the rest over the sea. Most were around and over the upper North Island.

'This front was coming down, and New Zealand, particularly Northland and Auckland, is quite a skinny landmass, and the front extended to the west and east of that area. There was lightning right along it,' MetService meteorologist Peter Little said.

Lightning that struck the Sky Tower on Wednesday was among 4000 over the New Zealand region in a 24-hour period.

Of those over the land, 230 were over Northland, 110 over Auckland, 379 over Bay of Plenty, 37 over Coromandel Peninsula, and 28 over Waikato. There were also a few over Fiordland.

* Watch: Auckland Sky Tower hit by lightning

* Lightning strikes 30,000 times and now for tornados**

The total was only about a tenth of the number of strikes recorded on the busiest days, but it was still a decent number, Little said

'That was still a lot of lightning over the upper North Island, and pretty wild weather - strong winds, heavy rain, hail and tornados,' he said.

'There was a period of very heavy rain in Auckland. The sky would have been very black. Things would have gone quite dark in the middle of the day - thunder and lightning as well. So Aucklanders would have all noticed it.'

Lightning tended to be more common in the upper North Island in summer, Little said.

At this time of year most of the lightning was on fronts that moved up the South Island. 'Fronts generally remain fairly strong until they get to around Taranaki and start to weaken.' In summer, fronts often weakened as they moved up the West Coast.

At warmer times of year fronts can be caused by convergence lines. That's when sea breezes coming from different directions meet.

'Where the air comes together, it has to go somewhere. In the atmosphere, when air converges at low-levels, it gets pushed upwards,' MetService explained in a blog on its website.

'Provided that there's enough moisture in the air, the uplift will generate cloud and potentially precipitation too.' If the air was unstable, thunderstorms could be generated.

Little said lightning in the upper North Island at this time of year was more likely to happen on fronts. 'It's more likely than the other type because it's getting colder but it doesn't happen that often,' he said.

Statistics New Zealand data shows the country received about 187,000 ground and sea lightning strikes a year between 2001 and 2016. That was for an area that covered the land and about 500km out to sea.

Parts of the West Coast in the South Island had the highest annual average density of ground strikes, up to 40 per 25 square km a year.

Parts of the Taranaki, Waikato, and Bay of Plenty regions were prone to lightning, with an annual average of up to 16 ground strikes per 25sq km a year in some areas.